Hi Ryan, I just closed off a support case today with a similar issue for HTML5 Blast on BigIP 13.1
We also got HTML5 Blast working by changing the view.proxy_addr session variable, we actually deleted it entirely which again directs the Blast protocol to initiate to the FQDN name of the VIP (it keeps using the running browser session URL).
We are running "unsupported" versions (old View iApp 1.2, BigIP 13.1, Horizon 7.3.2) so I assumed our Blast issue was due to changes in how the iApp/BigIP VDI code interacts during the setup wizard. I haven't got to test a new deployment of 13.1 with iApp 1.5.3 yet.
Supported versions are at https://support.f5.com/csp/article/K15041
Is your AP carried over from a previous BigIP/iApp version, or did you recreate the APM on version 13.1/iApp 1.5.3?
Cheers,
Josh
PS. During the 12.0 to 13.1 upgrade, we also had to manually add a /Common/vdi{} line before the BigIP config would load.